1. Field of the Invention
The present invention relates to a backlight unit for a key of an input device provided for a wireless phone, a telephone, a portable digital assistant (PDA), a keyboard, and a calculator, and more particularly, to a backlight unit with a light guide plate having a characteristic in which bending is easy and having a light guide pattern portion in which a plurality of engraved or embossed patterns are formed in a plurality of requirement character shapes on a predetermined regular basis.
2. Description of the Related Art
In general, a backlight unit which is used for a non-emissive flat panel display such as a liquid crystal display (LCD), keypad backlights of a telephone and a wireless phone, or light signboards is disclosed in Korean Patent Application Nos. 1997-73783, 1998-30060, 1999-47621, 2000-63111, and 2001-73468, and Korean Utility Model Application No. 1996-47524. In the disclosed backlight unit for a key of an input device, a plurality of light emitting diodes (LEDs) are directly disposed at each necessary part of bottom surfaces of keys to generate local light only at the necessary part, or a light emitting plate such as an organic electro luminescence light emitting diode (OLED) is installed.
In a method for directly disposing the plurality of LEDs at the bottom surfaces of the keys, the LEDs are required in number as many as the keys. Accordingly, this method has many drawbacks in cost and power consumption. In order to overcome these drawbacks, a method for suitably disposing LEDs in space between keys and lighting two or three keys by one LED without installing the LED at each key is used. However, this method has a lethal disadvantage in that constant luminance required at each key is not provided and luminance dispersion between the keys is greatly caused to thereby deteriorate product quality, and requires still many LEDs less than number of the keys.
Even in a method in which, instead of a LED, a surface light-emitting plate device such as an organic electro luminescence (EL) is disposed on a top surface of a point-contact switch, and a key is again disposed on a top surface of the resultant, there are several drawbacks such as a drawback of light efficiency of the device itself and a drawback of increase of a manufacture cost and endurance in a repeated operation.
In order to overcome the drawbacks, in a conventional backlight unit of a liquid crystal display (LCD) panel including a transparent light guide plate for transmitting light and a light emitting unit disposed on a side surface of the light guide plate, an edge light emitting method is widely used owing to its advantageous high efficiency and low price. However, in this method, since the light guide plate is of general plastic resin not bent, it is impossible that the light guide plate is used for a backlight unit of a key of an input device such as a telephone and a wireless phone. In other words, since a keypad used for a telephone and a wireless phone has a flexible bottom considering its function, when the key is pressed, the key should be pressed to the bottom to activate a point-contact switch and therefore, the light guide plate cannot be used in a conventional light guide plate method.
In the backlight unit for the key of the input device, since light emission is performed not over a total surface but over a local surface, there is a drawback in that a front light guide pattern method based on the conventional edge light emitting method is not performed at suitable efficiency.
Accordingly, in order to employ the edge light emitting method in the backlight unit of the key of the input device, it is required to use, instead of conventional solid material, flexible and transparent resin, such as relatively flexible silicone, as material of the light guide plate, and use a pattern using a particular method for enhancing light efficiency.
However, in this case, there are additional drawbacks in that it is impossible to employ a pattern installation method such as a conventional printing method due to a silicone adhesive property of an ink pattern, and it is impossible to process silicone in a scratch method due to softness of the silicone.
In a conventional method in which, in case where injection using a metal mold is applied to manufacture of the flexible light guide plate, minute protrusions are densely disposed on a surface of the light guide plate, there is a drawback in that thickness is relatively very small compared to a total area and it is difficult to obtain luminance enough to backlight the key of the input device that should concentrate light emission only on a specific character shape.
Meantime, as shown in FIG. 1, in a conventional backlight unit using an edge light emitting method, there is an advantage in that, when a pattern densely disposed over a whole surface of the light guide plate 10 is used, a backlight needing a general surface light source such as a liquid crystal display can equally obtain sufficient luminance from the whole surface. However, if such surface light emission is applied as it is to the backlight unit of the key of the input device such as the telephone and the wireless phone, light generated from other parts than a necessary light emitting part such as a character part on the key are all lost. In other words, there occurs a drawback in that effectively obtainable luminance is merely a very little compared to a total amount.
In order to overcome this drawback, as shown in FIG. 2, the pattern densely disposed over the whole surface of the light guide plate 10 can be grouped as a predetermined group not over the whole surface, and can be disposed in an island shape. However, in this case also, light emission can be reduced at a part other than a required position, but an amount of emitted light is not suddenly increased at a pattern part required for light emission. This is caused by the following reason. When incident light from a light emitting unit 20 passes through the light guide plate 10 and comes in contact with a light guide pattern, the incident light is diffused or scattered. In case where the light guide pattern is constructed in a plurality of island shapes forming a shape of a whole key terminal, though the light emission is generated from the shape of the whole key terminal, an amount of light capable of being actually recognized by a user through a character is limited by a shape and an area of the character.
In particular, in the light guide plate 10 for backlighting the key of the input device, a very little thickness of 500 μm or less in its product structure is required in comparison to a thickness of 1 mm or less of a conventional wireless phone light guide plate. In order to secure flexibility, the light guide plate 10 has a special property using silicone having a light transmittance of 90% or less even when compared to polymethyl methacrylate having a light transmittance of 98% or more which is material of the conventional light guide plate. Therefore, light is smoothly transmitted within the light guide plate 10. Accordingly, number of times the light emitted from the light emitting unit 20 is reciprocated within the light guide plate 10 is suddenly reduced while probability of absorbing the light within the light guide plate 10 is increased.
In other words, a design of a pattern shape allowing the light emitted from the light emitting unit 20 to pass through the opening portion 31 of a character shape at a possible high efficiency is required.
In such design direction, when the island shape is based on the character shape not a simple pattern of the key terminal shape, the probability in which the light emitted from the light emitting unit 20 passes through the opening portion 31 of the character is increased.
In case where the light guide pattern having the island shape is designed to have a size or a shape of a type accurately consistent with the shape of the opening portion 31 of the character, an amount of diffused and scattered light is suddenly reduced, thereby rather deteriorating total luminance.
In a product including the keys having a plurality of various characters, in case where the opening portion 31 area of the character is smaller than those of other keys, even though the opening portion 31 of the character itself provides the same level of luminance, there occurs a phenomenon in which, when a user recognizes the luminance at a predetermined distance with naked eyes, he/she feels darker than when a character portion having the small opening portion 31 is small. Accordingly, a method of luminance control inversely proportional to the opening portion 31 of the character is additionally required.